Editing Stirling engine (section)

== Operational considerations ==
[[File:Sterling engine small clear.ogv|thumb|Video showing the compressor and displacer of a very small Stirling Engine in action]]

=== Size and temperature ===
Very low-power engines have been built that run on a temperature difference of as little as 0.5 K.<ref name="Senft-1996" /> A ''displacer-type Stirling engine'' has one piston and one displacer. A temperature difference is required between the top and bottom of the large cylinder to run the engine. In the case of the ''low-temperature-difference'' (LTD) Stirling engine, the temperature difference between one's hand and the surrounding air can be enough to run the engine.<ref name="Romanelli-2020" /> The power piston in the displacer-type Stirling engine is tightly sealed and is controlled to move up and down as the gas inside expands. The displacer, on the other hand, is very loosely fitted so that air can move freely between the hot and cold sections of the engine as the piston moves up and down. The displacer moves up and down to cause most of the gas in the displacer cylinder to be either heated, or cooled.{{citation needed|date=July 2020}}

Stirling engines, especially those that run on small temperature differentials, are quite large for the amount of power that they produce (i.e., they have low [[power density|specific power]]). This is primarily due to the heat transfer coefficient of gaseous convection, which limits the [[heat flux]] that can be attained in a typical cold heat exchanger to about 500&nbsp;W/(m<sup>2</sup>·K), and in a hot heat exchanger to about 500–5000&nbsp;W/(m<sup>2</sup>·K).<ref name="Organ-1997" /> Compared with internal combustion engines, this makes it more challenging for the engine designer to transfer heat into and out of the working gas. Because of the [[thermal efficiency]] the required heat transfer grows with lower temperature difference, and the heat exchanger surface (and cost) for 1&nbsp;kW output grows with (1/ΔT)<sup>2</sup>. Therefore, the specific cost of very low temperature difference engines is very high. Increasing the temperature differential and/or pressure allows Stirling engines to produce more power, assuming the heat exchangers are designed for the increased heat load, and can deliver the convected heat flux necessary.

A Stirling engine cannot start instantly; it literally needs to "warm up". This is true of all external combustion engines, but the warm up time may be longer for Stirlings than for others of this type such as [[steam engine]]s. Stirling engines are best used as constant speed engines.

Power output of a Stirling tends to be constant and to adjust it can sometimes require careful design and additional mechanisms. Typically, changes in output are achieved by varying the displacement of the engine (often through use of a [[swashplate]] [[crankshaft]] arrangement), or by changing the quantity of working fluid, or by altering the piston/displacer phase angle, or in some cases simply by altering the engine load. This property is less of a drawback in hybrid electric propulsion or "base load" utility generation where constant power output is actually desirable.

=== Gas choice ===
[[File:Stirling Engine 1min NCTU.webm|thumb|Video of a bench top stirling engine demonstrating the speed and power.]]

The gas used should have a low [[heat capacity]], so that a given amount of transferred heat leads to a large increase in pressure. Considering this issue, helium would be the best gas because of its very low heat capacity. Air is a viable working fluid,<ref name="Organ-2008b" /> but the oxygen in a highly pressurized air engine can cause fatal accidents caused by lubricating oil explosions.<ref name=Hargreaves /> Following one such accident Philips pioneered the use of other gases to avoid such risk of explosions.
* [[Hydrogen]]'s low [[viscosity]] and high [[thermal conductivity]] make it the most powerful working gas, primarily because the engine can run faster than with other gases. However, because of hydrogen absorption, and given the high diffusion rate associated with this low [[molecular weight]] gas, particularly at high temperatures, H<sub>2</sub> leaks through the solid metal of the heater. Diffusion through [[carbon steel]] is too high to be practical, but may be acceptably low for metals such as [[aluminum]], or even [[stainless steel]]. Certain ceramics also greatly reduce diffusion. [[Hermetic seal|Hermetic]] pressure vessel seals are necessary to maintain pressure inside the engine without replacement of lost gas. For high-temperature-differential (HTD) engines, auxiliary systems may be required to maintain high-pressure working fluid. These systems can be a gas storage bottle or a gas generator. Hydrogen can be generated by [[electrolysis]] of water, the action of steam on red hot carbon-based fuel, by gasification of hydrocarbon fuel, or by the reaction of [[acid]] on metal. Hydrogen can also cause the [[hydrogen embrittlement|embrittlement]] of metals. Hydrogen is a flammable gas, which is a safety concern if released from the engine.
* Most technically advanced Stirling engines, like those developed for United States government labs, use [[helium]] as the working gas, because it functions close to the efficiency and power density of hydrogen with fewer of the material containment issues. Helium is [[inert gas|inert]], and hence not flammable. Helium is relatively expensive, and must be supplied as bottled gas. One test showed hydrogen to be 5% (absolute) more efficient than helium (24% relatively) in the GPU-3 Stirling engine.<ref name="Thieme-1981" /> The researcher Allan Organ demonstrated that a well-designed air engine is theoretically just as ''efficient'' as a helium or hydrogen engine, but helium and hydrogen engines are several times more ''powerful per unit volume''.
* Some engines use [[air]] or [[nitrogen]] as the working fluid. These gases have much lower power density (which increases engine costs), but they are more convenient to use and they minimize the problems of gas containment and supply (which decreases costs). The use of [[compressed air]] in contact with flammable materials or substances such as lubricating oil introduces an explosion hazard, because compressed air contains a high [[partial pressure]] of [[oxygen]]. However, oxygen can be removed from air through an oxidation reaction or bottled nitrogen can be used, which is nearly inert and very safe.
* Other possible lighter-than-air gases include [[methane]] and [[ammonia]].

=== Pressurization ===
In most high-power Stirling engines, both the minimum pressure and mean pressure of the working fluid are above atmospheric pressure. This initial engine pressurization can be realized by a pump, or by filling the engine from a compressed gas tank, or even just by sealing the engine when the mean temperature is lower than the mean [[operating temperature]]. All of these methods increase the mass of working fluid in the thermodynamic cycle. All of the heat exchangers must be sized appropriately to supply the necessary heat transfer rates. If the heat exchangers are well designed and can supply the heat [[flux]] needed for convective [[heat transfer]], then the engine, in a first approximation, produces power in proportion to the mean pressure, as predicted by the [[West number]] and [[Beale number]]. In practice, the maximum pressure is also limited to the safe pressure of the [[pressure vessel]]. Like most aspects of Stirling engine design, optimization is [[multivariable calculus|multivariate]], and often has conflicting requirements.<ref name="Organ-1997" /> A difficulty of pressurization is that while it improves the power, the heat required increases proportionately to the increased power. This heat transfer is made increasingly difficult with pressurization since increased pressure also demands increased thicknesses of the walls of the engine, which, in turn, increase the resistance to heat transfer.{{citation needed|date=July 2020}}

=== Lubricants and friction ===
[[File:STM Stirling Generator set.jpg|thumb|A modern Stirling engine and generator set with 55 kW electrical output, for combined heat and power applications.]]

At high temperatures and pressures, the oxygen in air-pressurized crankcases, or in the working gas of [[hot air engines]], can combine with the engine's lubricating oil and explode. At least one person has died in such an explosion.<ref name="Hargreaves" /> Lubricants can also clog heat exchangers, especially the regenerator. For these reasons, designers prefer non-lubricated, low-[[coefficient of friction]] materials (such as [[Rulon (plastic)|rulon]] or [[graphite]]), with low [[normal force]]s on the moving parts, especially for sliding seals. Some designs avoid sliding surfaces altogether by using diaphragms for sealed pistons. These are some of the factors that allow Stirling engines to have lower maintenance requirements and longer life than internal-combustion engines.{{citation needed|date=July 2020}}